Magneto-impedance (MI) is a classical electromagnetic effect occurring in all soft magnetic conductors where a strong skin effect is present [1]. The skin depth depends on the magnetic permeability and resistivity of the conducting material and on the frequency, xcfx89, of the current flowing through the conductor. For higher frequency and permeability, the cross-section of the conductor traversed by an a.c. current diminishes; this is equivalent to an increase in the wire impedance. This current also produces a circumferential magnetic field, which magnetizes the conductor in a thin layer under the surface, orienting its spines in a circumferential direction with respect to the wire axis. When a longitudinal d.c. magnetic field, H, is applied to the wire, the spins will rotate toward this field direction and the circumferential permeability changes. The skin depth, and thus the wire impedance, are changed. The relative change in the wire impedance, which can now be viewed as depending on the static field, via permeability, constitutes the MI effect, which is usually defined as [2]:
Z(xcfx89,H)/Z(xcfx89,H)=[[Z(H)xe2x88x92Z(Hsat)]/Z(Hsat)]xc3x97100 (%). 
Near-zero magnetostriction magnetic wires have already been used for magnetic field sensing. Related art describes magneto-impedance sensing elements made of FeCoSiB amorphous wires prepared by quenching a jet of molten material in a rotating layer of water, known as xe2x80x9cUnitikaxe2x80x9d wires [3]. For example, U.S. Pat. No. 5,994,899 describes such an element, having near-zero magnetostriction, which is supplied with a dc-biased alternating current to produce a voltage drop across the element, said voltage drop depending on the strength of an applied magnetic field. In such embodiment, the properties of the sensing element are well defined after casting and are not tunable for particular sensitivity. The fine tuning is normally provided by complex electronic circuitry.
Magneto-transport effects in metals are normally non-linear around zero magnetic field, and their response is usually a parabola centered around the field origin. Thus, in order to linearize the operation of any sensor using these effects, said parabola must be displaced with its linear part crossing the zero field axis. This operation is known as the biasing of the sensing element. Bias magnetic fields may be provided by coils wound around the element (U.S. Pat. No. 6,121,770), or by permanent magnets located in the vicinity of the element (U.S. Pat. No. 6,097,183). Moreover, biasing may occur as a result of thermo-mechanical treatments used to induce or change specific magnetic properties of the material forming the sensing element. For example, Unitika wires were found to exhibit a spiral magnetic anisotropy (the micro-magnetic equivalent of linearization) and a magneto-impedance vs. field sensitivity of 10%/Oe following the twisting of the wire (U.S. Pat. No. 5,994,899).
U.S. Pat. No. 6,069,475 describes a magneto-impedance sensor having as sensing element a short-circuited micro-strip device having a soft magnetic wire as the upper conductor and a copper layer as the ground conductor. The magneto-impedance of two soft wires is used in a multi-vibrator-type of electronic circuit to measure the gradient of a magnetic field, as described in the U.S. Pat. No. 5,831,432.
Related art also describes amorphous wires cast by other methods than quenching in rotating water. For example, U.S. Pat. No. 5,003,291 describes melt extraction, a casting method where wires are extracted by a rotating wheel from a melted alloy. The molten material adheres to the wheel and quickly solidifies. This technique generates quenched-in tensile stresses which follow the radial directions of the temperature gradients during solidification [4]. In negative magnetostriction wires, the coupling of radial stresses to the magnetic spins induces a circumferential easy axis (the direction of minimum energy of spins) in the outer shell of the wire. Due to demagnetization, the easy axis is longitudinal in the inner core of the wire. The magnetization of the wire occurs through spin rotation in the outer shell and wall displacement in the inner core, the latter being associated to large Barkhausen jumps in the hysteresis loop [5]. The magnetic structure of melt-extracted wires is different from that of xe2x80x9cUnitikaxe2x80x9d wires. Moreover, twisting and/or annealing of melt-extracted wires were found to significantly diminish the magneto-impedance effect in these wires.
We have discovered that magnetic sensors, based on the magneto-impedance effect and made of stress-sensitive, negative-magnetostriction amorphous wires, cast by melt extraction, exhibit unexpected and useful properties.
An object of an aspect of the present invention is to provide a simple method to obtain a sensing element with significantly enhanced sensitivity and, at the same time, to control the magnitude of said sensitivity.
An object of another aspect of the present invention is to provide a simple and practical method of internal linearization of the operation of said sensing element, thus eliminating the need for complex electronics and/or external biasing magnetic fields.
An another object of yet another aspect of the present invention is to obtain a memory element, with memory of its previous saturating magnetic state, based on the magneto-impedance effect in stress-sensitive, negative-magnetostriction amorphous wires, cast by melt extraction.
More particularly, this invention provides a magnetic field sensor comprising, as sensing element, a wire made of a stress-sensitive material, wherein said wire is submitted to a longitudinal tensile stress.